The atomic nucleus - a new world with new forces

Nucleons

The atomic nucleus consists of protons
with positive charge, which were discovered in 1919 by Rutherford, and neutrons that are electrically neutral, as
discovered by Chadwick in 1932. The protons and neutrons (or nucleons as
they are both called) are kept together by a new kind of
force that we do not encounter in our everyday life, the
so called strong force. It is called so because it is so
strong that it can keep the atomic nucleus together even
though the protons are repelled by each other due to the
electromagnetic force. With the help of only three building
blocks, the proton, the neutron, and the electron, as well
as their electromagnetic and strong interactions we can form
all the atoms in the periodic system.

Protons and neutrons are equally affected
by the strong force, but not all particles are. One such
example is the electron. The particles that are affected
by the strong force are called hadrons and today we know
of several hundred different kinds of hadrons. Today we also
know that the hadrons are not fundamental particles, but
instead they are built up of quarks. The first proofs that
the proton has an inner structure came in the end of the
1960's from an experiment done at Stanford Linear Accelerator
Center in the USA. By irradiating protons in a fixed target
with high energy electrons and studying how the electrons
were scattered one concluded that protons have an inner structure,
the quarks. Already earlier one had hints that protons and
neutrons have an inner structure based on there magnetic
dipole moments which do not at all agree with what one expects
from a pointlike particle. For the proton one had measured
the electric dipole moment to be 2.79 instead of 1 and for
the neutron -1.91 instead of 0.

The structure of the atomic nucleus

In the same way as the electrons surrounding
the atomic nucleus are in different energy levels or shells,
the nucleons in the atomic nucleus are also in different
shells. Among the first to study the energy levels of the
atomic nucleus was Helge Tyrén in Uppsala. Together
with his collaborators he studied the energy required to
knock out protons from the atomic nucleus. The results confirmed
a theory for the shells of the atomic nucleus which had been
put forward earlier. At the The Svedberg Laboratory in Uppsala
one is still doing research on the shell structure of different
atomic nuclei and collective phenomena such as vibrational
and rotational excitations.

The Gustaf Werner
cyclotron was built in the late 1940's and was for a
short while the most powerful accelerator in Europe.
It has since then been upgraded and is today used for medical and
nuclear physics research. It is also used as an injector
for the so called CELSIUS ring, where research is done
both in nuclear and particle physics.